Many high-volume consumer products such as mobile devices and motor vehicles now include a digital camera. For example, FIG. 1 shows a mobile device 190 having a camera module 180 integrated therein. Camera module 180 includes an image sensor 102 beneath an imaging lens 170.
FIG. 2A is a perspective view and FIG. 2B is a cross-sectional view of a chip-scale package (CSP) 200 that includes image sensor 102. The cross-sectional view of FIG. 2B is in a plane parallel to the y-z plane of coordinate system 298. CSP 200 includes a substrate 210, an isolation layer 220, an electrically conductive redistribution layer (RDL) 240, a top solder mask layer 250, and solder ball pads 261 and 262. For clarity of illustration, top solder mask layer 250 is omitted from FIG. 2A. FIGS. 2A and 2B include RDL segments 240(1-2), which are part of RDL 240. Substrate 210 is formed of silicon for example, and isolation layer 220 may be formed thereon via plasma-enhanced chemical vapor deposition. Isolation layer 220 may be regarded as part of substrate 210. Isolation layer 220 has a top surface 220T.
Solder ball pads 261 and 262 are electrically connected to RDL segments 240(1) and 240(2), respectively. Solder ball pad 261 and a portion of RDL segment 240(1) form a power pin 261P configured to be electrically connected to a voltage source. Solder ball pad 262 and a portion of RDL segment 240(2) form a ground pin 262G.
A problem with current CSPs is that they are prone to failure in humid environments, as evidenced by failures in temperature-humidity-bias testing. In a humid environment, surface insulation resistance decreases between adjacent conductors, such as RDL segments 240(1-2), which results in electrochemical migration and current leakage or short-circuiting of the power and ground pins. For example, metal species 265 (such as nickel) migrates to top surface 220T in an isolation layer region 225, in which top surface 220T functions as a leakage-current path between pins 261P and 262G.